This application relates generally to oscilloscopes and more particularly to an oscilloscope for use in time resolving an ultrafast voltage signal.
Many photoelectric devices, most notable of which are the photomultiplier tube and the streak camera, have been used in the past to time resolve very short optical pulses.
Basically, a photomultiplier tube comprises a photocathode, an electron multiplier and an anode, all disposed in an evacuated glass housing, with potential differences set up between the electrodes and the electron multiplier to cause photoelectrons emitted by the photocathode when it is illuminated to pass through the electron multiplier and on to the anode.
In the operation of a photomultiplier tube a beam of light strikes the photocathode, causing a number of electrons, the number being proportional to the intensity of the incident light beam, to be emitted into the evacuated housing. These electrons are then multiplied by the electron multiplier to produce a stronger signal and, thereafter, transmitted through the housing to the anode where the electrons are collected to produce an output voltage signal.
Because of the electron multiplication, photomultiplier tubes are especially well adapted among photosensitive devices currently used to detect radiant energy in the ultraviolet, visible, and near infrared regions. Photomultiplier tubes also feature relatively fast time response.
The photocathode in a photomultiplier tube is generally arranged in either a side-on or a head-on configuration. In the side-on type configuration the photocathode receives incident light through the side of the glass housing, while, in the head-on type, light is received through the end of the glass housing. In general, the side-on type photomultiplier tube is widely used for spectrophotometers and general photometric systems. Most of the side-on types employ an opaque photocathode (reflection-mode photocathode) and a circular-cage structure electron multiplier which has good sensitivity and high amplification at relatively low supply voltage.
The head-on type photomultiplier tube has a semitransparent photocathode (transmission-mode photocathode) deposited upon the inner surface of the entrance window while in the side-on type, the photocathode is a separate structure. Because the head-on type provides better uniformity and lower noise, it is frequently used in scintillation and photon counting applications.
The electron multiplier in a photomultiplier tube is usually in the form of either a series of electrodes, called dynodes, or a microchannel plate. As is known, a microchannel plate (MCP) is a form of secondary electron multiplier consisting of an array of millions of glass capillaries (channels) having an internal diameter ranging from 10 um to 20 um fused into the form of a thin disk less than 1 mm thick. The inside wall of each channel is coated with a secondary electron emissive material having a proper resistance, and both ends of the channel are covered with a metal thin film which acts as electrodes, thus each channel becomes an independent secondary electron multiplier.
When a voltage is applied between both sides of the MCP, an electric field is generated in the direction of the channel axis. When an electron hits the entrance wall of the channel, secondary electrons are produced. These secondary electrons are accelerated by the electric field and travel along the parabolic trajectories determined by their initial velocity. Then they strike the opposite wall and produce other secondary electrons. This process is repeated many times along the channel, and, as a result, the electron current increases exponentially towards the output end of the channel.
The photocathode in a head-on type photomultiplier tube is generally circularly shaped and in a side-on photomultiplier tube is usually in the shape of a portion of a cylinder.
One of the limitations of photomultiplier tubes is that although they have a relatively fast time response, they are not capable of time resolving luminous events in the picosecond time frame. On the other hand, an optoelectric device that does have the capability of time resolving luminous events in the picosecond time frame is the streak camera.
Streak cameras are over fifteen years old in the art and have been used, hitherto, to directly measure the time dynamics of luminous events (i.e. to time resolve a light signal). A typical streak camera includes an entrance slit which is usually rectangular, a streak camera tube, input relay optics for imaging the entrance slit onto the streak camera tube, appropriate sweep generating electronics, electron accellerating means, and output-relay optics for imaging the streak image formed at the output end of the streak camera tube onto an external focal plane. The image at the external local plane is then either photographed by a conventional still camera or by a video camera. The streak camera tube generally includes a photocathode, an accelerating mesh, sweeping electrodes, and a phosphor screen. The streak camera tube may also include a microchannel plate. Light incident on the entrance of the streak camera is converted into a streak image from the start of the streak to the end of the streak corresponding to the intensity of the light incident thereon during the time window of the streak. The time during which the electrons are swept to form the streak image is controlled by a sweep generator which supplies a very fast sweep signal to the sweeping electrodes. The input optics of the streak camera may comprise a single lens.
U.S. Pat. No. 4,659,921 to Alfano, a streak camera which can be gated on and off over an ultrashort time window, such as in picoseconds or femtoseconds, is disclosed. The device includes, in one embodiment, an input slit for receiving a light signal, relay optics, a sweep generator and a tubular housing, the tubular housing having therein a photocathode, an accelerating mesh, a pair of sweeping electrodes, a microchannel plate, a variable aperture and a dynode chain. Light received at the input slit is imaged by the relay optics onto the photocathode. Electrons emitted by the photocathode are conducted by the accelerating mesh to the sweeping electrodes where they are swept transversely across the tubular housing at a rate defined by the sweep generator over an angular distance defined by the sweeping electrodes, in a similar manner as in a typical streak camera. Swept electrons strike the microchannel plate where electron multiplication is accomplished. Exciting electrons which pass through the variable aperture and which strike the first dynode (cathode) in the dynode chain are further multiplied and outputed from the last dynode anode in the dynode chain as an analog electrical signal, the analog electrical signal corresponding to the intensity of the light signal during the time window over which swept electrons are picked up by the first dynode. In another embodiment of the invention all of the dynodes in the chain except for the last dynode are replaced by a second microchannel plate.
In U.S. Pat. No. 4,467,189 to Tsuchiya, a streak camera is disclosed which includes a cylindrical airtight vacuum tube, a shutter plate, and a ramp generator. The container has a photocathode at one end thereof and a flourescent screen at the other end thereof which is opposite to the photocathode. The shutter plate is disposed between and parallel to the surface of the photocathode and fluorescent screen and has a multiplicity of through holes perforated perpendicular to its surface. The shutter plate also carries at least three electrodes that are disposed perpendicular to the axis of the through holes and spaced parallel to each other. The electrodes divide the surface of the shutter plate into a plurality of sections. The ramp generator is connected to the electrodes. The ramp voltage generated changes in such a manner as to reverse its polarity, producing a time lag between the individual electrode. Developing an electric field across the axis of the through holes in the shutter screen, the ramp voltage controls the passage of the electron beams from the photocathode through the through holes. A framing camera includes the above-described framing tube and an optical system. The optical system includes a semitransparent mirror that breaks up the light from the object under observation into a plurality of light components and a focusing lens disposed in the path through which each of the light components travels. Each of the light components corresponds to each of the sections on the shutter plate. The images of a rapidly changing object are produced, at extremely short time intervals, on different parts of the fluorescent screen.
As can be appreciated, the streak camera devices described above are useful only for time resolution of optical pulses.
In copending application by Alfano et al., Ser. No. 091,123, filed Aug. 31, 1987, there is disclosed a photomultiplier tube constructed for use in either time resolving an ultrafast test voltage signal or time resolving an ultrafast optical pulse. The photomultiplier tube comprises a housing having therein a photocathode for receiving incident light and producing emission of electrons in proportion to the intensity of the light, the photocathode having a transmission strip line configuration, an accelerating mesh for accelerating electrons emitted by the photocathode, a microchannel plate for performing electron multiplication on the electrons emitted from the accelerating means, an anode for receiving electrons from the microchannel plate and producing an analog electrical signal output, a power supply for use in applying a biasing voltage across the photomultiplier tube so that electrons emitted by the photocathode will be conveyed through the accelerating mesh and the microchannel plate and onto the anode, and cables connected to the photocathode for receiving and transmitting an ultrafast voltage signal.
It is an object of this invention to provide a new and improved oscilloscope.
It is another object of this invention to provide an oscilloscope for use in time resolving an ultrafast test voltage signal.
It is a further object of this invention to provide an oscilloscope as described above which includes a streak camera.
It is still a further object of this invention to provide an oscilloscope as described above which includes a specially designed photomultiplier tube.
It is yet still a further object of this invention to provide an oscilloscope as described above for use in time resolving an ultrafast voltage signal with minimal background noise.
It is another object of this invention to provide a photomultiplier tube having a photocathode in the form of transmission stripline whose width is matched to the output slit size of a spectrometer whose output is detected by the photomultiplier tube or matched to the cross sectional size and shape of a fiber optics bundle connecting the spectrometer to the photomultiplier tube.